Lithium secondary batteries are used as power supplies loaded on electrically-driven vehicles, or as power sources installed in PCs and other mobile terminals, etc. In particular, lightweight lithium-ion secondary batteries with high energy densities are becoming increasingly important as high-power power supplies loaded on vehicles (e.g. power sources for driving motors connected to drive vehicle wheels).
A typical lithium secondary battery includes a separator which separates a positive electrode active material layer and a negative electrode active material layer. The separator prevents a short circuit associated with direct contact between these two active material layers, and with the pores of the separator being impregnated with an electrolyte solution, it also serves to form ion-conducting paths (conduction channels) between the two electrodes. Typical examples of a separator used in a general lithium secondary battery include a single-layer or multi-layer porous sheet formed with thermoplastic resin (typically with a polyester or a polyolefin such as polypropyelene, etc.). A separator formed from such a porous resin sheet may provide, in addition to the short-circuit prevention, a feature (shutdown function) to block ion-conducting paths when the internal temperature of the battery is elevated to a certain temperature range (typically the melting point (or softening point) of the separator) so as to stop charging and discharging to prevent a further temperature increase. The ion-conducting paths are blocked when fine pores of the separator shut as it melts or soften.
A separator formed from a porous resin sheet as described above undergoes thermal contraction when the temperature reaches a point where the shutdown function is activated. When a great deal of thermal contraction occurs, a local tensile stress may be applied to the sheet to open a hole or the surface area of the sheet may shrink, possibly causing the two electrodes to contact each other to form a new short-circuit spot. Regarding a heat-resistant separator having an inorganic oxide porous membrane on a surface of organic porous film, Patent Document 1 states that the heat-resistant separator provides a shutdown function by means of the organic porous film and it also maintains insulation by means of the inorganic oxide porous membrane even when the organic porous film melts down.